Air/fuel control systems are known which are responsive to exhaust gas oxygen sensors positioned both upstream and downstream of the catalytic converter such as shown in U.S. Pat. No. 5,115,639. It is also known to use a ratio in switching transitions of the upstream and downstream sensors to provide an indication of converter efficiency such as shown in U.S. Pat. No. 5,357,751.
The inventors herein have recognized numerous problems with the above approaches when catalytic converter monitoring is applied to variable displacement engines. For example, when operating an 8-cylinder engine with only 4 cylinders combusting under light engine load, variations in manifold pressures and exhaust gas pulsations between the 4 VS. 8 cylinders modes results in a different frequency of transitions in the output of the exhaust gas oxygen sensors. Accordingly, prior approaches to monitoring the efficiency of the converter would provide inaccurate results when groups of engine cylinders are disabled.